This invention relates generally to perceiving acceleration upon an object. More specifically the invention relates to the devices used for making such perceptions, known as accelerometers. In greater specificity, the invention relates to an optical accelerometer system created through the technology known as micro-electro-mechanical systems or xe2x80x9cMEMSxe2x80x9d.
In the above U.S. patent application, there is disclosed a micro-electro-mechanical ultra-sensitive optical accelerometer. This device has been coined the name MEMS USA by its inventors. The MEMS USA embodiment disclosed as an example in the cited patent application is based upon the monolithic integration of an interferometer and a photodiode. Such an accelerometer is shown generally in FIG. 1.
FIG. 1 illustrates in a simplified depiction how a Fabry-Perot optical resonator is created by using two reflective mirrored surfaces that are partially transmitting and that are separated by a distance, d. If monochromatic light is illuminated onto the upper mirror and the distance between the mirrors is an integral multiple of half wavelengths of the monochromatic light, then a resonant condition will exist within the cavity created by the mirrors. A light transmission peak through the bottom mirror will occur.
If the distance between the mirrors is not an integral multiple of half wavelengths of the incoming light, then destructive interference will occur within the cavity and the transmission will be exponentially attenuated.
In the optical accelerometer disclosed in the referenced patent application, the mirror distance is initially adjusted to maximize transmission of light though the cavity. One mirror, such as the top mirror, is used as a proof mass and is hinged or supported in such a way to allow movement with respect to the other mirror due to an external force such as acceleration. Any displacement in the upper mirror will therefore cause a decrease the light transmission through the cavity. The photodiode shown in FIG. 1 is monolithically integrated with the Fabry-Perot cavity so that a change in transmitted light is easily detected and recognized as a change in the optically generated current Ipd of the photodiode. Thus, by monitoring the photodiode current, a measure of acceleration can be obtained.
FIG. 2 illustrates such a combination of Fabry-Perot cavity 10 and photodiode 12. In this instance an air-silicon interface makes up lower mirror 14, although as explained in the cited patent application, other mirror configurations can be used such as dielectric stacks. Upper mirror 16 consists of either a partially transmitting dielectric stack or a thin layer of metal on a transparent or semitransparent membrane, for example.
FIG. 3 shows the measured and predicted output for the device of FIG. 2. In this instance an electrostatic force (Vm) is used to vary the distance between upper mirror 16 and lower mirror 14. The plot in FIG. 3 shows the change in photocurrent generated by the photodiode as the electrostatic force (Vm), and hence the distance between the mirrors, is varied. Multiple peaks occur in the transmitted light due to the existence of several resonance points within the particular tested system and for the initial airgap distance of the fabricated device.
FIG. 4 shows a biasing scheme by which the MEMS USA accelerometer of the previously cited patent application generates a family of curves similar to that of a metal-oxide-field-effect-transistor (MOSFET), illustrating that the device possesses xe2x80x9ctransistor-likexe2x80x9d characteristics. In the accelerometer field, this amplification characteristic can be highly advantageous, as all prior art accelerometers detect a displacement that is then fed to the input of an amplifier, leading to unwanted noise. The MEMS USA becomes an amplifier itself whose output can be used as the input stage of a low noise amplifier, greatly reducing noise.
In this configuration, photodiode voltage (Vpd) designates the reverse bias regime such that the diode is turned xe2x80x9conxe2x80x9d for negative voltages. Here voltage Vpd is used to bias the pn photodiode and is swept (increased or decreased) under reverse bias conditions. By using bias voltage Vm to keep the force across the mirrors constant, one of the curves shown in FIG. 5 is created. Adjusting the force across the mirrors and again sweeping bias voltage Vpd across the photodiode results in a second distinct curve. Thus by discretely changing the force exerted on the upper mirror and sweeping the photodiode voltage, a family of curves can be generated.
The MEM USA device thus appears to take the characteristics of a three terminal device with the upper mirror acting like the gate electrode of a MOSFET. However, unlike the MOSFET, minority carriers in the form of photo-generated electron hole pairs are introduced into the depletion region of the pn photodiode much like that of a bipolar junction transistor (BJT).
By biasing the optical accelerometer with an ideal load in the form of a current source and monitoring the voltage across the photodiode, a mechanism to amplify small displacements in the upper mirror due to a perturbing force can be realized.
In the immediate invention, the MEMS USA device of above-referenced patent application is arranged in one or more pairs of cooperating accelerometer cells whose outputs are differentiated (output difference is ascertained). This configuration provides for even greater accelerometer sensitivity than a single independent accelerometer cell, and allows for a reduction in common mode noise due to amplitude and phase difference variations of the utilized light source as well as supply voltage or any other noise common to the pair of accelerometers. The differential approach of the invention provides for the biasing of the MEMS-based optical accelerometers such that their output signals are 180 degrees out of phase with each other.
Other objects, advantages and new features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanied drawings.